New evidence for water ice and organics on Mercury

A thermal map of the north polar region of Mercury. The blue areas denote permanently shadowed regions cold enough for water ice to be stable on or below the surface.

Planetary scientists have identified water ice and anomalously dark deposits within permanently shadowed regions at Mercury’s north pole. Using data collected by NASA’s MESSENGER spacecraft, a UCLA team crafted the first accurate thermal model of the solar system’s innermost planet that successfully pinpoints extremely cold regions where ice has been found on or below the surface. They conclude that the newly discovered black deposits are a thin dark crust of residual organic material brought to the planet over the past several million years by water-rich asteroid and comet impacts.

Understanding how water ice has been preserved on Mercury and where it came from may help scientists determine the conditions necessary for sustaining life on other planets.

This research, one of three MESSENGER papers published online today and in an upcoming print edition of Science, sheds light on the longstanding issue of ice on Mercury. The sun-scorched planet is now revealed to have extensive water ice deposits at its poles by several independent lines of evidence.

In the early 1990s, scientists were surprised to find that areas near Mercury’s poles were unusually bright when observed with radar from Earth, a potential indication that ice might be present.

Lead author David Paige, a self-described “professional ice finder,” has studied the poles of planetary bodies in the solar system from Mercury to Pluto.

“Mercury is the innermost planet in the solar system and arguably it’s among the least explored,” said Paige, a professor of Earth and space sciences at UCLA. “The surface of Mercury exhibits the most extreme range of temperatures of any body we know of in the solar system.”

Within a single polar crater on Mercury there are places that reach the oven-like temperature of 500 degrees Fahrenheit within sight of areas cold enough to freeze and preserve water ice for billions of years. These “natural freezers” exist within shadowed areas inside polar crater rims that never experience direct sunlight due to the low angle of the Sun at such high latitudes, Paige said.

Paige was able to use the first detailed topographic map of Mercury’s north polar region produced by MESSENGER to generate an accurate thermal model of the pole. His calculations of Mercury’s subsurface temperatures are a near perfect match to Earth-based radar observations and surface brightness measurements made by the Mercury Laser Altimeter (MLA) instrument onboard the orbiting spacecraft.

Where his temperature model predicts water ice should be stable on the surface, the MLA nearly always measures unusually bright patches, indicative of surface ice deposits. In places where it is too warm for surface ice but cold enough for ice to exist beneath the surface, the MLA sees unusually dark material.

“This stuff we find covering the ice is darker than the rest of Mercury, which is already a really dark planet. That’s amazing,” said Paige. “At the very least it means there is something out of the ordinary going on inside these permanently shadowed areas where the ice has accumulated.”

The mysterious dark substance likely arrived on Mercury as part of comets and asteroids that periodically crash into the planet, bringing water ice and a diverse cocktail of organic material, Paige said. In the searing daytime heat of Mercury, the only place water and organics can survive is within permanently shadowed craters.

But only in the very coldest areas of the permanently shadowed regions can water ice exist on the surface. In the warmer shadowed areas, the top layers of ice begin to evaporate away into space, leaving behind a layer of hardy organic molecules that are stable at higher temperatures and turn black over time when exposed at the surface. Once the dark layer is thick enough, it protects the ice underneath, allowing a subsurface ice deposit to survive.

“There are areas on the surface where it is too hot for ice to exist, but radar data from Earth show something bright reflecting from these areas so we’re pretty sure that there’s water ice buried underneath,” said co-author Dr. Matthew Siegler, a JPL researcher and recent UCLA alumnus. “You need some kind of insulating layer to keep that heat from getting down to the ice.”

The presence of bright ice and dark organics on Mercury’s surface presents a mystery for MESSENGER researchers. Large comets and asteroids periodically impact Mercury, covering a huge swath of the planet in a layer of dirt and dust and adding yet another crater to the airless planet’s scarred landscape. For the water ice and black organic layers to remain exposed on Mercury’s ancient surface, the deposits must have formed recently in Mercury’s geological history or must be maintained by new water brought to Mercury by smaller, more frequent impacts.

“Billions of years ago, the Earth acquired a layer of water and other volatile material that formed atmospheres, oceans, and even the first organic molecules that started life,” said Paige. “Understanding the origin of that material is a very important problem and is essential to finding out about the potential habitability of planetary systems around other stars.”

Other UCLA co-authors include Ellen Harju, a graduate student in the department of Earth and space sciences. Paige’s study was published alongside two other MESSENGER papers with colleagues David Lawrence and Greg Neumann as the lead authors. All three research discoveries were showcased today in a press conference on NASA TV.

Launched in 2004, MESSENGER became the first spacecraft to orbit Mercury in March of 2011. Previously, humankind’s closest glimpse of the innermost planet in the solar system was during three flybys of Mercury by the Mariner 10 spacecraft in 1974–75. The name MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry and Ranging, was chosen to evoke the Greco-Roman messenger deity Mercury, a god of trade, merchants, and travel.

Professor David Paige explains how a thermal model at UCLA helped determine where water ice is found on Mercury.

Professor David Paige discusses how water ice may have arrived on Mercury and the origin of mysterious dark material measured by MESSENGER.

Professor David Paige explains why studying ice on Mercury is important for understanding the origin of life on Earth and the potential habitability of extrasolar planets.

The online version of the print edition (published on January 17, 2013) is available here. And don’t forget to learn how David Paige crafted the cover of Science magazine to showcase the recent MESSENGER discoveries.